[KS Cleaning]

I have natural stone slabs in my back garden that are in need of a good clean, they are a bit discoloured and are starting to get black spots around the edges, can someone recommend a good product that will remove these stubborn marks, I will be using a bosch electric power washer. Also after I have cleaned them is it advisable to have them sealed?

[APWS]

the black spots a likely to be lichen, not sure about the pressure washer though could do with a turbo try hypo

[KS Cleaning]

Thanks for replying,is there some chemical that would help remove the lichen or would a good pressure wash alone do the job?